PROJECT SUMMARY/ABSTRACT Priapism is defined as prolonged penile erection occurring unassociated with sexual interest. 40% of male sickle cell disease (SCD) patients display priapism. The disorder is dangerous and urgent given its association with erectile tissue damage and erectile dysfunction. Current strategies to manage the disorder are poor due to lack of fundamental understanding of the pathophysiology of priapism. Recently my laboratory unexpectedly found that male adenosine deaminase (ADA)-deficient mice display features of priapism seen in humans, including spontaneous prolonged penile erection associated with increased vascular relaxation in response to neurostimulation with subsequent penile fibrosis. In addition, we demonstrated that reducing the accumulation of adenosine by ADA enzyme therapy relieved spontaneous prolonged penile erection and corpus cavernosal strip relaxation both in vivo and in vitro. Moreover, the analysis of four adenosine receptor deficient mice revealed that the A2B adenosine receptor (A2BR) is essential for adenosine-mediated penile relaxation and erection and that upregulated A2BR signaling contributes to priapic activity in ADA-deficient mice. Finally we found that priapic activity in the SCD transgenic mouse, a well accepted priapic animal model, is also due to elevated adenosine signaling via A2BR, suggesting a general contributory role of adenosine and A2BR signaling in priapism. Thus, in two independent lines of mutant mice, one with ADA deficiency and another with SCD, we have shown that excessive adenosine accumulation in the penis, coupled with increased A2BR signaling, contributes to priapism. We now propose to extend these observations by determining the cellular targets and signaling pathways associated with adenosine-induced priapism, the molecular mechanism of increased adenosine production in priapism and the general role of adenosine signaling in the initiation and maintenance of normal penile erection. The following specific aims are proposed. I. What are the intracellular targets and signaling pathways involved in adenosine-induced priapism? II. What are the molecular mechanisms generating excess adenosine in priapism? III. What are the sources of adenosine in the penis and what regulates its production during initiation and maintenance of normal penile erection? Specific Aims I and II represent a continuation of our efforts to understand the molecular basis of adenosine- induced priapism. Specific Aim III represents an expansion of these efforts to understand the role of adenosine signaling in normal penile erection. As a result of our ongoing research activities we have available all the necessary lines of mutant mice as well as relevant research experience in areas of biochemistry, molecular genetics and physiology needed to conduct the research proposed here. Proposed research is likely to reveal an important role for adenosine signaling in several aspects of the penile erection process and highlight various therapeutic opportunities to treat priapism and other erectile disorders.